I am investigating the control of cell division, my specific goals being the identification of the genes involved, the elucidation of their molecular products, and ultimately the determination of the physiological roles that these play. Yeast (Saccharomyces cerevisiae), a simple eukaryotic organism amenable to both genetic and biochemical investigations, will be employed for the proposed research. In yeast, division is normally controlled in response to nutrient limitation and in preparation for conjugation. Both forms of control are integrated in the G1 interval at a step of the cell cycle known as start. Undoubtedly many gene products are directly involved and elucidation of their individual roles would be a major step in clarifying how control of cell division in eukaryotes is organized. In order to identify the relevant genes, mutants which cannot complete the start step were sought. To date, I have isolated 40 independent mutations and assigned them to four unlinked complementation groups. The procedure to be used to identify the start gene products, will be the molecular cloning of the start genes by transformation of mutant yeast cells, the use of cloned genes to isolate complementary mRNAs and the translation of these RNAs in vitro to identify the gene products. Once the start gene products are identified, antisera will be prepared against them to be used as polypeptide-specific immunological probes. Both the metabolism and intracellular locations of the start gene products will be investigated.